1. Field of the Invention
This invention pertains to apparatus for generating a supersonic jet of gaseous ions.
This invention also pertains to a filtration system comprising a supersonic jet ionizer for imparting electric charge to particulates in a gas stream, and a filter medium disposed downstream of the ionizer for electrostatically removing charged particulates from the gas stream.
2. State of the Prior Art
An apparatus for generating a sonic jet of gaseous ions was described in U.S. Pat. No. 3,317,790. Use of a sonic jet ionizer for imparting electric charge to particulates in a gas stream in order to enhance the filtration efficiency of a downstream filter medium was described in co-pending U.S. patent application Ser. No. 061,978 filed on July 30, 1979 now abandoned.
In a typical jet ionizer, an electric field is established between an elongate high-voltage electrode and a grounded metallic plate, with the high-voltage electrode being mounted in a gas-receiving chamber and the grounded plate serving as a wall of the chamber. This geometric configuration produces an electric field of sufficient strength to establish a corona in the region adjacent the high-voltage electrode. The gas-receiving chamber is provided with an inlet to admit pressurized gas; and the grounded plate is provided with an aperture to enable the gas to flow as a jet from the chamber. The high-voltage electrode is positioned with respect to the aperture in the grounded plate so that the gas passes through the corona in exiting from the chamber. The corona generates ions in the gas passing therethrough; and the gaseous ions so formed are attracted, depending upon their polarities, toward either the high-voltage electrode or the edge of the exit aperture in the electrically grounded plate. The velocity of gas flow through the corona in a typical jet ionizer is sufficient to cause a significant number of the ions formed in the corona to be swept out of the chamber through the exit aperture before they can reach the high-voltage electrode or the grounded plate.
A jet of gas can be produced by allowing the gas to expand through an aperture from a region of higher pressure into a region of lower pressure. If the gas is allowed to pass from the high-pressure region into the low-pressure region through a nozzle having a flow channel that is much longer than the flow path would be through a simple orifice, a well-formed jet of very high-velocity gas can be obtained. Definitions of the terms "nozzle" and "orifice" as used in the art, and a theoretical discussion of gas flows through nozzles and orifices, are provided in the text entitled Thermodynamics by George Hawkins, Wiley Publishing Co., New York, 1951, (Second Edition), pages 313 et seq.
Jet ionizers have been used in basic research for size distribution studies of charged particulates. Practical applications of jet ionizers in industry have generally involved neutralizing electrically charged particles. However, the advantage of a supersonic gas flow from a jet ionizer have not heretofore been recognized in industrial applications; and a practical device for generating a supersonic jet of gaseous ions has not heretofore been available.